1. Field of the Invention
The present invention relates to a television tuner and, more particularly, to a television tuner for balance-inputting a television signal to a balanced mixer.
2. Description of the Related Art
FIG. 5 shows a structure of a conventional television tuner. Balanced input ports 51 and 52 are connected to a tuning coil 53, and a serial circuit composed of a variable-capacitance diode 54 and a capacitor 55 is connected in parallel to the tuning coil 53. Further, a tuning DC-voltage input port 56 for controlling the capacitance of the variable-capacitance diode 54 is connected to a connection point between the variable-capacitance diode 54 and the capacitor 55 through a resistor 57. The above-mentioned structure is a primary side structure.
As a secondary side structure, a tuning coil 61 is connected to a double balanced mixer 20 through capacitors 62 and 63. In addition, a serial circuit composed of a variable-capacitance diode 64 and a capacitor 65 is connected in parallel to the coil 61. Also, a tuning DC-voltage input port 66 for controlling the capacitance of the variable-capacitance diode 64 is connected to the connection point between the variable-capacitance diode 64 and the capacitor 65 through a resistor 67. Further, one end of a resistor 68 is connected between the variable-capacitance diode 64 and the tuning coil 61 and the other end of the resistor 68 is grounded.
In addition, a predetermined frequency signal output from a local oscillator 14 is mixed to the output of the tuning circuit to make a predetermined intermediate frequency signal, and the intermediate frequency signal is supplied to intermediate frequency output ports 21, 22 (For example, see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 07-007452 (FIG. 1)
In the above-mentioned structure, as a means for balance-inputting the television signal to the double balanced mixer, a balanced tuning circuit is provided. However, as the tuning circuit used in a television tuner, an unbalanced tuning circuit including a single tuning circuit and a tuning circuit having a simple structure and a stable characteristic is mainly used.